The invention relates to a method for measuring the flow velocity of a medium while applying a magnetic field to the measured volume fed through. In particular, the invention relates to such a method which also allows the use of temporally constant magnetic fields.
The principle of magnetic-inductive flow or throughflow measurement technology has proved itself to be outstanding in practice. It has many advantages in comparison with other measurement principles; for example, the measured values are independent of density, viscosity and—within certain limits—also of the flow profile and the conductivity of the medium, as long as the conductivity exceeds a minimum value of approx. 1 μS/cm. Additionally, sensors working according to this principle can also dispense with any constrictions or dead spaces in the measured volume. They are therefore also suitable for the measurement of dirty liquid streams which are loaded with solids. They can also be used for aggressive and corrosive fluids in the case of suitable selection of the electrically insulating inner wall lining and the type of the signal tapping.
For the most part, galvanic signal tapping is used, in which the sliver- or spherical-cap-shaped metallic electrodes, which are provided using through passages through the pipe inner wall and are of small diameter (generally a few millimeters), are directly in (galvanic) contact with the medium. They must be provided with reliable seals with respect to the pipe wall. This type of tapping stands out on account of a robust and simple setup, but is susceptible to chemical attack, deposits and abrasion at the electrodes and the seals. The control of high operating temperatures requires a careful adjustment with regard to the thermal coefficient of expansion of the electrode material and of the wall material.
These disadvantages can be circumvented by capacitive signal tapping. The electrodes are in this case not in contact with the medium, which, for its part, only comes into contact with the pipe inner wall. The design object is thus reduced to the selection of a sufficiently resistant inner wall material.
The indicated advantages of magnetic-inductive throughflow sensors—together with the two modes determined by the type of signal tapping—are responsible for the large scope of application of the magnetic-inductive measurement principle. These advantages are confronted with the disadvantage of a substantial energy outlay for the creation of the magnetic field, however. Both modes require temporally changeable magnetic fields, in order either—as in the case of galvanic signal tapping—to eliminate disruptive electrochemical potentials or, for principal physical reasons,—as in the case of capacitive signal tapping with permanently specified coupling capacitances—to achieve signal tapping at all. These fields can be generated with just one electromagnet. In the case of the magnetic-inductive throughflow and flow sensors known today, the energy requirement for the creating the magnetic field is therefore almost always covered from the electrical energy network.
This stands in contrast to the current trend for future development in sensor technology, however, which is characterized by a great interest in what are known as “energy-autarchic” systems, which cover their energy requirement from a network-independent energy source (battery, accumulator, solar cells, etc.) and in the process should achieve a service life of at least five years.
It is therefore obvious that a measuring method which is suitable in the sense of energy autarchy can only be configured convincingly in accordance with the magnetic-inductive principle if it can be based on the use of permanent magnets alone.
Corresponding solution suggestions already exist for flow and throughflow sensors with capacitive signal tapping.
Thus, it is suggested, for example in German published patent application DE 102 21 677 A1 to replace the temporally changeable magnetic field with a permanent magnetic field and the previously permanent coupling capacitances with coupling capacitances which can be controlled at a time interval, which can be predetermined, and for their part allow a capacitive signal tapping in the same time interval.
An alternative is described in German published patent application DE 10 2005 043 718 A1, in which the signal tapping is provided by controllable semiconductors—preferably field effect transistors (FETs)—on whose gates, which gates are provided with an insulating layer, the induced voltage acts directly, in that the gates are in contact with the measured medium. This practically currentless measuring method avoids the otherwise necessary charging of a capacitor in order to evaluate its charging as a measure for the flow velocity. It can nonetheless likewise be construed as a capacitive method, however, in which the voltage to be measured at the gate electrode simulates precisely this charging. The advantage lies in the fact that even small changes in the flow velocity entail an easily measurable signal in a permanent magnetic field.
It is therefore not amazing at first glance that the measured signals of a measurement setup based on a permanent magnetic field and FETs as signal tapping at a constant flow velocity are a superposition made up of a temporally constant level value with a temporally changeable slight fluctuation about the level value. Signals of this type are known per se from the very first MIDs, in which permanent magnets and galvanic tapping were used. These fluctuations can be suppressed using measurement technology or computationally eliminated in a number of known ways.
Upon reflection, the person skilled in the art may, however, see that the physical causes for the fluctuations in the two types of tapping can be significantly different. For example, random variations of the local charge distribution for the signal at the FET, which ultimately detects a spatially averaged electric field, are not very important, whilst this would have a considerable influence on the ideally punctiform galvanic electrodes. Even other material effects, particularly impurities, are in the case of FET tapping nowhere near as important as in the case of a galvanic tap.
The singly certain common cause of the signal fluctuations in both cases is the disruption of the fluid movement due to turbulence. As even in the case of intrinsically laminar flow of the fluid, small fluctuations of the flow velocity occur, e.g. due to the roughness of the pipe inner wall or the like.
Hereinafter, reference shall be made to individual results of turbulence research (see e.g. J. G. M. Eggels: “Direct and Large Eddy Simulation of Turbulent Flow in a Cylindrical Pipe Geometry,” Delft University Dissertation 1994, Delft University Press, ISBN 90-6275-940-8).
It is known from that that turbulence phenomena stretch across entire cascades with one another by energy transfer of coupled eddies which decrease in terms of their size from the largest dimensions determined by the flow geometry to the smallest dimensions which are found in the region of intermolecular spaces.
In the case of pipes with the internal diameter D, the largest eddies are characterised by a typical length scale L[m] of the size L≈0.1 D. The energy necessary to maintain the eddy is taken from the flow in that it is the flow that drives the largest eddy. By continued transfer along the energy cascade from large to smaller and smaller eddies, this energy is finally dissipated on what is known as the Kolmogorov scale, that is to say is converted to heat.
In addition to this length scale, there is a typical velocity scale, which is designated as u [m/s]. It characterizes the fluctuating velocity. In the wake of this, there appears a typical time scale which is given by the quotient L/u [s]. It can be interpreted as the typical lifetime of an eddy. Its reciprocal value u/L [1/s] describes a typical frequency scale. From this, there follows a typical energy (per unit mass) of u2 [m2/s] and a typical (average) energy dissipation rate (per unit mass), which is given by the quotient of energy and lifetime ε=u2/(L/u)=u3/L.
Interestingly, the average dissipation rate ε is not dependent on the microstructure of the medium, that is to say on its molecular properties—for example its viscosity. It is rather determined solely by the flow itself, that is to say by the flow geometry (also obstacles, surface condition, etc.) and by the average flow velocity.
This statement is of fundamental importance for turbulence theory: It states that turbulence is a property of the media flow, not however of the medium.